Process for preparing photographic gelatin



1937- s. E. SHEPPARD E! N. 2,101,877

V PROCESS FOR PREPARING PHOTOGRAPHIC GELATIN Filed Sept. 6, 1935 ATTORNEY. y

Patented Dec. 14, 1937 raocnss roa PREPARING rno-rooaarmc earn-rm Samuel E. Sheppard and James H. Hudson,

Rochester, N. Y., assignors, by mesne assignments, to Eastman Kodak Company, Jersey City, N. J a corporation of New Jersey Application September 6, 1935, Serial No. 39,455

4 Claims.

This invention relatestoa process of producing gelatin for photographic uses.

In the usual procedure for obtaining photographic gelatin from hides, the hides are first 5 washed with water to remove dirt and foreign matter and are then soaked in saturated limewater which generally contains a considerable amount of free lime. This removes the hair, and

this depilatory process is also accelerated by the 1 formation of sulfides by inter-action of lime with young hair substances. The fats are next saponified and converted to lime soaps. After this the hide is subjected to a swelling in alkaline solution.

whereby the soluble albumens difiuse out, to-

15 ether with other soluble substances which are photographically active and tend to cause fogging in the sensitive emulsions produced from the gelatin. This swelling in the alkaline bath for plumping may require from one to several 20 months. The stock is then partially neutralized with an aqueous acid and the gelatin extracted by treating with water.

The gelatins-produced in this way exhibit a general uniform type of colloid behavior. These 25 properties are at a decided minimum at a hydrogen ion concentration of about 4.9. This minimum also corresponds to the iso-electric point a of such gelatins. It has been found that although the duration of extraction, the concentration of 3 the gelatin at the time of setting to a jelly, and

other, minor variations affect the colloid properties to some extent, these variations are, however, secondary and do not prevent gelatins obtained by alkaline treatment from exhibiting the further object to provide a method for making a photographic gelatin having reduced swellingproperties in alkaline solution and possessing 50 other colloid properties different from those of gelatin obtained by alkaline treatment of hides.

In the accompanying drawing, Figure"1 ..;is a.

graphic representation'of the. colloid properties of gelatin obtained by alkaline treatment of hides.

5 Figure 2 is a. graphic representation of the colloidl ing photographic gelatin, the hides, such as calfskin, are first washed to remove dirt or foreign matter and are chopped into pieces of approxi- 'mately uniform size, for example, of about 8 to 10 square inches. These pieces are then soaked in an excess of dilute acetic acid. The concentration of this acid may vary from to 2%, depending upon the age and condition of the stock. Dln'lng treatment in the acid bath, the

hide pieces are agitated from time to time to insure complete action by'the acid. In about six or eight days the hair, still embedded in the exterior cuticle or epidermis, will loosen and may be removed from the hide by rubbing or agitation.

The partly swollen residual hide pieces are then left in the acid'for a suflicient time for full plumping to take place. This action may be completed in a week, or it may require a. somewhat longer time.

After plumping, the acid liquor is drained from the stock and the'hide pieces are then neutralized to a pH? of six to seven. This is carried out in I the following manner:v The hide pieces are placed in an aqueous solution and the solution adjusted to the desired'pH by addition of an'alkaline ma- I terial. After the hide pieceshave remained in this solution for an appreciable time, one of the pieces is removed and sliced through the middle to obtain an exposed section; The exposed section is coated with an'indi'cat'or solutionwhich indicates pH of "the hide pieccupon comparison of the color with a' standard] chart. If the hide piece shows that the center of -the piece has not reached the pH of the solution, the soaking of the remaining pieces is continued until testing in thisv manner shows that the centers of the pieces-have reached the. desired degree of neutralization. Indicators which may be used to determine the pH of the hide pieces in this way are Brom Cresol green which indicates pH in the range of pH 3.8 to 5.6, Chlorphenol red which indicates pH in the range of pH 5.2 to 6.8, and Brom thymol blue which indicates pH in the range of pH 6 to 7.6. Color nuance may be estimated with these dyes with an accuracy of pH .2 to .4.

Extraction of the gelatin is then carried out in the usual way by treating with water at a temperature of 1'70 to 180 F. A large part of the fat remains in the hides with the residual elastin priorto extraction of the gelatin, but most of it ing these colloid properties are grouped together and represented by the letter F. The letter F therefore represents a composite of the units in which the various colloid properties are expressed. For example, the swelling in water is determined by the increase in weight when the gelatin is placed in water for an appreciable length of time. In this case. the letter F would represent the weight of the gelatin when placed in water at the various units of pH. In a similar manner change in. viscosity in aqueous solution is measured by the time required for the solution to pass throughan. orifice o1 definite size. The time required at various units of pH would, therefore, be represented by the letter F. The precipitation from solution in alcohol is measured as the amount precipitated from a solution of definite concentration upon addition of alcohol to. thesolution. The amount precipitated would, therefore, be represented by the letter F in' the present case. The curves obtained on plotting each of these properties separately against the pH'are found to show a similar general outline. If these properties are plotted against the pH of a gelatin solution, it is found that, for gelatin obtained by alkaline treatment-of hides, there is a sharp minimum in the curve at pH 4.9. This is represented by the curve A in Figure 1. For gelatin obtained by acid treatment of hides according to our process, the colloidproperties when plotted We have described our process with particularreference to the use of acetic acid in treating the hides. However, we are not limited to the use of this acid, any weakly-ionized acid other thanacetic bc-in'g adaptableto our process. The treatment of the hides with such acids is extremely mild and the'hides areexposed tohydrolysis for a much shorter time than in alkaline treatment, and for this-reason a very strong gelatin solution is obtained. We have referred to the treatment of calf skins by our process, but it is obvious that other hides might be treatedin a similar manner.

It will be seen, by reference to the curve shown in Figure 2 of the drawing, that gelatins obtained according to our process exhibit less swelling in alkaline solution than gelatines obtained byalkaline treatment of hides. This is illustrated by the flattened portion of the curve shown in the region of pH 7 onward. This property is of particular advantage in making photographic emulsions for use where alkaline conditions of processing are to be employed. An example of this is in tropical development where it is usually necessary to use a highly alkaline solution and to employ anti-swelling compounds to prevent excessive swelling of the gelatin. The use of our gelatin obviates the necessity of using such anti-swelling compounds in the development of highly alkaline solutions. Numerous other advantages are also obtained by the use of our acid-extracted gelatin.

What we claim and desire to secure by Letters Patent of the United States is:

l. A process of preparing photographic gelatin having a minimum region of colloidal properties, such as swelling in water and change in viscosity in alkaline solution, at pH 7 and upward, which comprises steeping, unhairing, and plumping hides in about a to 2% solution of the weakly ionized acid, neutralizing'the hides to a pH of about 6 to '7, and extracting the gelatin from them in water solution. I

2. A process of preparing photographic gelatin having a minimum region of colloidal properties, such as swelling in water and changein viscosity in alkaline solution, at pH' '7 and upward, which comprises steeping, unhairing, and plumping hides in about a to 2% solution of acetic acid, neutralizing the hides to a pH of about 6 to 7, and extracting the gelatin from them inwater solution.

3. A process of preparing photographic gelatin I having a minimum region of colloidal properties,

such as swelling in water and change in viscosity in alkaline solution, at pH '7 and upward, which .comprises steeping, unhairing, and plumping calfskin in a dilute solution of a weakly ionized acid,

neutralizing the hides to a pH of about 6 to 7 and soluneutralizing the calfskin to a pH of about 6 to 7 and extracting the gelatin from it in water solution. SAMUEL E. SHEPPARD. JAIVIES H. HUDSON. 

